You can open multiple websites at once, either by opening a new Safari window (File > New Window) or by creating a new tab within the current Safari window (File > New Tab).
You can even switch views if you change your mind. Simply drag a tab out of a window to create a new window for the page you’re viewing. Or, choose Window > Merge All Windows to create a single window with multiple tabs for each open webpage.
Tip: Command-click any link to open it in a new tab or window.